1. Field of the Invention
The present invention relates to a protective layer forming device which forms a protective layer containing a protective agent on a surface of a photoconductor, and also relates to an image forming apparatus using the protective layer forming device, and a process cartridge using the protective layer forming device.
2. Description of the Related Art
In an image forming apparatus utilizing electrophotographic process, image formation is carried out by subjecting a photoconductor to a charging step, an exposing step, a developing step and transferring step. Subsequently, discharge products which are generated in the charging step and remaining on the photoconductor surface, and toner residues or toner components remaining on the photoconductor surface after the transferring step are removed through a cleaning step.
As a cleaning system commonly used in the cleaning step, a rubber-made cleaning blade which is less expensive and superior in cleanability and has a simple mechanism is used. The cleaning blade, however, is press-contacted to a surface of a photoconductor so as to remove residues on the photoconductor surface, and thus a large mechanical stress is caused by friction between the photoconductor surface and the cleaning blade, the cleaning blade is abraded, in particular, in an organic photoconductor, its surface layer is abraded, undesirably, shortening the operating life of the cleaning blade and the organic photoconductor (OPC).
Toners for use in image formation have become smaller in size in response to demands for obtaining higher quality of images. In an image forming apparatus using such a toner having small particle diameter, the toner residues frequently pass through a cleaning blade, and in particular when the dimensional accuracy of a cleaning blade and assembling accuracy of the cleaning blade are insufficient or a part of the cleaning blade vibrates, toner slippage occurs at very high rate. This problem has been preventing the formation of high-quality images.
In order to prolong the operating life of an organic photoconductor and maintain high image quality, it is necessary to reduce deterioration of members, such as cleaning blades and photoconductors, caused by friction therebetween, and to improve the cleanability.
To meet the above-mentioned demands, for example, Japanese Patent Application Publication No. (JP-B) No. 51-22380 proposes a method in which a brush etc. is pressed against a protective agent block containing zinc stearate and the like to be micronized, the micronized protective agent is supplied onto a photoconductor, and a protective layer is formed by use of a cleaning blade.
This proposal is very favorable, because with use of a metal soap such as zinc stearate makes it possible to improve the lubricity of the photoconductor surface, reduce friction between the photoconductor and the cleaning blade and to improve the cleanability of the photoconductor surface, i.e., removability of untransferred toner residues.
Recently, in a charging step, a so-called AC charge has become frequently used, in which an alternating voltage is superposed on a direct voltage for charging a photoconductor surface by a charging roller. This AC charge causes fewer occurrences of oxidized gasses such as ozone and NOx, and has excellence in that a charging device can be downsized. In contrast, several hundreds times per second to several thousands times per second of negative/positive discharges are repeated between the charging member and the photoconductor, according to the frequency of the alternating voltage applied, and thus the surface layer of the photoconductor suffers from a number of discharges, and deterioration thereof is accelerated. To avoid the deterioration, a protective agent containing metal soap is applied onto a photoconductor, and the energy of AC charge is absorbed to the protective agent first, but hardly reaches the photoconductor, and thereby the photoconductor surface can be protected.
However, a protective agent containing metal soap is decomposed by energy of AC charge, but the metal soap is not completely decomposed to be distinguished, a fatty acid having low molecular weight is generated therefrom, and the frictional force applied between the photoconductor and the cleaning blade is easily increased. As a result, toner components easily adhere together with the fatty acid in the form of a film on the photoconductor, leading to low-image resolution, abrasion of the photoconductor, and nonuniformity of image density.
Therefore, it is necessary to supply a large amount of metal soap so that the photoconductor surface is immediately covered with metal soap, even if such a fatty acid is generated. With increased demands for higher speed performance in image formation, the linear velocities of photoconductors become increasingly faster, and in accordance with this, the amount of metal soap to be supplied to a photoconductor should be increased.
In general, only supplying micronized metal soap particles onto a photoconductor does not cover the photoconductor surface with uniformity, and thus it is necessary to press-spread micronized metal soap particles in the form of a film by means of a blade.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2005-275166, a metal soap is applied onto a photoconductor with the size of metal soap particles being set to 1 μm or smaller, to thereby improve the cleanability. However, it has become known that when the linear velocity of the photoconductor used is fast, small metal soap particles to be supplied to the photoconductor pass through a blade with vibration from the driven photoconductor, easily causing regions where the metal soap is not formed in the form of a film.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2008-224828 discloses that if metal soap particles to be supplied to a photoconductor are large in size, the metal soap reaches to the site immediately below a charging roller, metal soap particles electrostatically adhere onto the charging roller, the adhered metal soap is oxidized by the energy of AC charge to be fused and fixed to the charging roller. When the metal soap is fixed, it is fixed while involving toner components present on the photoconductor. Therefore, the resistivity of the charging roller at that part is increased to cause charging defects, resulting in the occurrence of black streaks. From the description above, it has been believed that metal soap particles to be supplied to a photoconductor has preferably small particle diameter.
Meanwhile, a protective agent block for use in an image forming apparatus is commonly produced by placing a melted metal soap into a molding dye, followed by cooling (see Japanese Patent Application Laid-Open (JP-A) No. 10-279998). Since crystal of the protective agent produced is isotropic and densely formed, and thus in order to increase the supply amount of metal soap, the pressure of a brush pressed against the protective agent block increases, and thus the durability of brush had been insufficient.
Also, metal soap particles scraped by a brush become an indefinitely shaped granular fine powder. The metal soap particles are blocked by a blade during low-linear velocity of the photoconductor, and then broken into pieces, further micronized and then applied onto the photoconductor. But when the amount of metal soap supplied in large amount, and the linear velocity of the photoconductor is made faster, relatively large metal soap particles before being applied onto the photoconductor are micronized to pass through a blade and reach a charging roller, then electrostatically adhere onto the charging roller, the adhered metal soap is oxidized by the energy of AC charge to be fused and fixed to the charging roller. When the metal soap is fixed, it is fixed while involving toner components present on the photoconductor. Therefore, the resistivity of the charging roller at that part is increased to cause charging defects, resulting in the occurrence of black streaks.
There is proposed a method for producing a protective agent block which causes no cracks and deficient portions, in which metal soap is placed in a dye which has been heated to a temperature lower than the melting point of the metal soap, 25° C. to 45° C., and compression-molded under reduced pressure (see Japanese Patent Application Laid-Open (JP-A) No. 2000-319224). According to this proposed method, energy for the temperature lower than the melting point of the metal sop is compensated by compression energy, and voids in the protective agent block are avoided by utilization of reduced pressure, and thus it is possible to produce a protective agent block which is substantially same as one produced by melting of a metal soap. However, in this protective agent block, when the amount of the metal soap is supplied in a large amount and the linear velocity of the photoconductor is made fast, relatively large metal soap particles before being applied onto the photoconductor are micronized to pass through a blade and reach a charging roller, then electrostatically adhere onto the charging roller, the adhered metal soap is oxidized by the energy of AC charge to be fused and fixed to the charging roller. When the metal soap is fixed, it is fixed while involving toner components present on the photoconductor. Therefore, the resistivity of the charging roller at that part is increased to cause charging defects, resulting in the occurrence of black streaks.
Further, as a metal soap, zinc stearate has been conventionally used, however, it has become known that use of a mixture of zinc stearate and zinc palmitate makes it possible to completely apply the mixture onto a photoconductor even in the case the linear velocity of the photoconductor is fast, but in the present situation, it is yet difficult to apply a mixture of zinc stearate and zinc palmitate onto a photoconductor, even when particles of the mixture are small or large.